1. Field of the Invention
The present invention relates to a high voltage generating device. The device according to the present invention is used as a spark ignition device for an internal combustion engine.
2. Description of the Related Art
A use of a piezoelectric (PZT) element as a spark ignition device for an internal combustion engine has been proposed by, for example, the Clevite Corporation of the United States (See E. Crankshaw et al. "A Piezoelectric Ignition System for Small Engines" SAE-D244, 1961 SAE Summer Meeting). In this use, a mechanical stress is applied externally to the PZT element by utilizing an eccentric cam. In other words, mechanical stress is externally applied to and an electric charge is generated from the PZT element, and if an electrode is provided, the electric charge can be extracted as a voltage. However, in this method, since it is relatively difficult to control the operation of the cam, spark timing cannot be adjusted, repeated discharges cannot be generated, and the energy discharged from the PZT element is relatively low. To counteract noise interference with radio frequency reception, a demand has arisen for a distributor-less ignition device. In Japanese Patent Application No. 59-42967, a piezoelectric type spark ignition device was proposed by the same applicant as the present invention.
When a columnar PZT element is stretched or contracted in a resonance state in the axial direction, an electric charge is generated in the same manner as under the application of a mechanical stress. This generates a high voltage across electrodes at two ends of the PZT element. To cause the element to resonate, a voltage, rather than a mechanical stress is applied to the element, in such a manner that the element itself comes under stress due to electrostriction. This principle has been also adopted for a PZT transformer. That is, an alternating voltage is applied across a pair of opposing electrodes so that resonance is caused by a strain component in a direction perpendicular to the direction in which a voltage is applied, and an electric charge generated thereby is trapped by an electrode, thus generating a high voltage. Although strain in the voltage application direction is large, since the PZT transformer utilizes strain in the direction perpendicular to the voltage application direction, an electrode having a large area is required in order to obtain a large electrostriction. Thus, the electrode occupies at least half of the area of the PZT transformer.